Field of the Invention
The invention relates to engagement and disengagement between a screw head and a screwdriver or drill shaft tip as the screw is driven into a work surface.
Discussion of Related Art
Screwdrivers of various forms are popular among homeowners, carpenters, builders, and almost anyone with a set of tools.
Most screws have similar features. The head of the screw contains a cavity of various shapes into which the tip of a screwdriver enters. The contact between the screwdriver tip and the head via this cavity enable rotation of the screwdriver to translate into rotation of the screw. The tip of the screw pierces a surface such as wood. The threads of the screw enable the screw to grip into the surface.
At a website for Home Improvement Stack Exchange, there is a discussion about techniques that help prevent slippage when driving a screw with a screwdriver. The following is an excerpt regarding five different kinds of screws:
Flat-head/slotted screws come in many sizes. Having a correctly-fitting bit helps a lot. Too narrow or too thin and you'll damage the head. Too wide and you'll damage the work. Too thick and it won't fit. Fingernails, coins, and knives are non-optimal. Make sure your bit is properly aligned in the slot. Keep the drill directly in line with the screw.
Phillip's head screws are actually designed to “cam out”. That is, when the screw stops turning easily, the bit is pushed up and out of the screw head. This is to prevent you from over-torquing the screw and damaging the work, screw, or bit. Unlike flat-head are discrete, #2 being the most common. Make sure you have a correct size. Keep the drill directly in line with the screw. Pressure on the drill is necessary to keep the bit in place. When the angle makes it difficult to apply pressure, set the clutch low and don't work too hard. When the clutch slips, turn the clutch up and apply more pressure to finish the work.
Pozidriv looks a lot like Phillip's, but has a subtly different shape that reduces cam-out. With clutches on drivers today, the chances of over-torquing are greatly reduced. Make sure you know if your bits and screws are Phillip's or Pozidriv. (Supadriv is very similar to Pozidrive.)
Torx, internal hex, and external hex are all easy to drive without much pressure and without cam-out. They also continue to work well if they get dirty or are painted over.
Square, aka Robertson, is easier to work with than Phillip's, but not as easy as Torx & hex heads.
Higher quality screws do not break or strip as easily as lower quality. Cheap screws are more likely to break or round out (i.e., strip) the head.
The following excerpt provides some practical advice:
Good-quality fasteners are worth it. Cheap screws are more likely to break or round out the head. If a driver bit slips out and damages the screw head, then you'll have a harder time finishing the work or removing the damaged screw. More torque means more damage if it slips, so be careful if you turn up the clutch. As soon as a screw is damaged happens, if you pull the screw out before it gets worse and replace it, you'll be better off than if you keep driving the bad screw.
An impact drill/driver makes driving screws much easier. The work turns to butter. They're also loud, a bit expensive, and can destroy your work if you're not careful Driving slowly lets you keep control and reduces damage when the bit slips.
Predrilling in metal/pilot holes in wood make it easier on your muscles, reduce screw breakage, reduce wood splitting, and don't reduce strength. I've heard that it may actually increase strength, but I don't know for sure. I pick a drill the size of the screw [stem].
If you're having trouble with just a few screws on something you want to look nice, you can always drill a pilot hole. This way you're just fighting the torque on the threads, rather than the torque required to displace wood around the shank and drive the threads.
Note that there are two kinds of screws, and hence two kinds of drill bits used for predrilling. Traditional screws taper evenly toward the tip, and should use a taper-point bit for their pilot holes . . . . Modern screws . . . have a constant “root diameter” until you get to the tip; pilot drills for those can be straight bits of that diameter or a bit smaller. (If you really want to do it right, you then drill a slightly larger section of pilot hole for the unthreaded portion of the shaft and, of course, countersink for the head.)
Soap can help lubricate screws in to wood, making it easier and reducing screw breakage.
You should set the speed to low, and use steady power. They sell cordless drills that have three essential features for this:
1. A really low speed setting.
2. A clutch. Whenever the screw is all the way in, it stops turning the drill.
3. Light weight; if you can't easily hold it in place, it's not going to work.
The following excerpt pertains to magnetic bit holders.
These can be helpful. They are typically magnetic, so they hold the screw in place. They also have a sliding sheath, so it will hold the screw in place until you have completely driven the screw.
I don't like the magnetic bit holders much, because it's easy to leave the bit in the fastener. The sheaths are nice when accuracy is unimportant, but if you want the screw to be really straight, you have to hold it a different way (or pilot).
After seeing a lot of amateurs that have a different concept of accuracy, I think these are fantastic for teaching the concept of lining the drill head up with the screw. And the magnets are nice when you're wearing gloves and can't pick the screw out of your pouch.
There are two sets of forces required to operate a screwdriver: (i) the rotational force to turn the screwdriver and screw and (ii) the forward directional force to hold the screwdriver tip in the screw head and to push the screw forward into the surface.
This standard process of using a screwdriver, however, comes with a negative side effect. When driving into a dense surface, such as pressure treated wood, the amount of forward force one needs to apply on the screwdriver can be quite high. As a result, it is common for the screwdriver to slip out of the cavity of the screw head, sometimes causing damage to the screw, screw head cavity, screwdriver, surface, wall, and/or user. This is such a well-known and common problem that products exist to remove screws whose head cavity has been stripped or damaged by screwdriver slippage.
When driving many screws into a dense surface, for example, even if using a powered screwdriver, the user must still exert considerable force onto the screw to prevent the screwdriver tip slipping out of the screw head, risking greater damage, physical injury and increased muscle fatigue. Some products attempting to reduce these issues are on the market, yet all such screwdriver products fail in high force applications.
For example, one such product uses a magnetized screwdriver tip. This product is of value only to screws that are attracted to magnets. The strength of the magnetic attraction between the screw and the screwdriver tip is almost always far weaker than the sheer force required to keep the screwdriver tip inside the screw head cavity when screwing into medium or hard surfaces. Hence, screwdriver slippage is not prevented.
Another such product uses springs in various configurations to keep the screwdriver tip connected to the screw head. This only works when the strength of the forward force required to keep the screwdriver tip inside the screw head cavity is less than the strength of the spring. Once again, hard and dense surfaces often require far greater force to keep the screwdriver tip in the screw head cavity than the springs can provide.
Another set of such products use various premolded forms with prongs that squeezes the screw. These premolded forms have some limitations such as the inability to accommodate various size screw heads, various shapes of the screw head, and various diameter screw stems, and the necessity of the devices to have to flex to insert the screw. As such, these devices suffer from the same consequences as the magnetized screwdriver tip.
Creative solutions of all sorts have been proposed to keep the screwdriver tip in the head of the screw. These include using tape, glue, and other adhesives. However, none of these solutions reduce the need for the user to maintain considerable forward force to ensure the screwdriver tip remains in the screw head.
For instance, in U.S. Pat. No. 2,762,409, tips are provided to hold a screw. However, each tip is affixed to a screwdriver and does not enable one to change screwdrivers. Further, the jaws have a more limited width of a screw head to which they can grip. The jaws are attached to long straight metal lengths. One would need to stretch these apart to enable a wide screw head to fit within these jaws.
U.S. Pat. No. 5,881,613, whose contents are incorporated herein by reference, provides in part:
A conventional power screwdriver is commonly used for driving a fastener, such as a screw, into various work surfaces. Such power screwdrivers do not provide a means for automatically stopping the rotation of a spindle which holds a driver bit. To use such a power screwdriver, an operator must know when to stop applying power to the motor with a trigger switch to stop the rotation of the motor. However, when a screw is driven into a delicate material, such as dry walls, a delay in disconnecting power to the motor may damage the work surface or may result in an excessive penetration of the screw into the work surface.
Some power screwdriver is equipped with a clutch mechanism to either transmit or disconnect the rotation force from the driver motor to the spindle. The clutch mechanism includes a fixed clutch connected to the driver motor and a movable clutch, which engages or disengages the fixed clutch in response to the pressure applied to a housing surrounding the driver bit when the housing is pressed against the work surface. During the disengaging operation, the separation of the clutches is usually abrupt and causes early wear of gear teeth. Similarly, when two clutches reengage each other, the gears or teeth of two clutches grind against each other to foster early wear.
There is a need for a device that enables one to insert a screwdriver through the center, holding the screwdriver tip in the screw head cavity with sufficient force so that when the screw drives through the surface of the material it is going into, the force required to go into the surface will not cause slippage. Such device will not strip the screw face cavity and must be able to hold each individual screw during the screw driving operation and then release the screw after it is screwed into the surface so that the device may then hold the next screw for the next screw driving operation.